Introduction
What is a railway traction transformer?
A railway transformer is designed for use in railway electrification systems to convert, isolate, and distribute electrical power for trains and related equipment. One of the main types is the traction transformer, which supplies power to the railway traction system.
A railway traction transformer, also known as a traction power transformer, typically converts high-voltage AC power from the utility grid or a railway power supply network into a voltage suitable for the traction system. It is commonly installed at a railway traction substation, where it supplies power to overhead catenary or third-rail systems. Some traction transformers are also designed for direct installation on electric trains.
Unlike conventional power transformers, a traction transformer must handle frequent and rapid changes in load. Train acceleration can cause sudden increases in current, while braking and changes in train operation can produce further variations in the electrical load. Therefore, railway traction transformers are designed for reliable operation under dynamic loads, electrical stresses, vibration, and other demanding railway conditions.
For a broader introduction to traction transformer construction and components
see our Traction Transformer Overview.
| 6MVA 22-1.22kV Railway Transformer | 5MVA 132-1.22kV Railway Transformer | 5MVA 88/2*1.22+2.36 kV Railway Transformer |
| 5MVA 66-1.22kV Railway Transformer | 5MVA 44-6.6 kV Railway Transformer | 5MVA 22-6.6 kV Railway Transformer |
Railway Transformer Working Principle
The railway transformer working principle is based on electromagnetic induction. In a traction substation, the transformer steps down high-voltage power from the grid or railway power supply network to a voltage suitable for the railway traction system.
High-Voltage Input: Electricity from overhead catenary lines or third-rail systems is fed into the primary winding of the railway transformer. The input voltage can range from 15 kV to 25 kV, depending on the railway system.
Voltage Transformation: The high-voltage input energizes the primary winding and creates a changing magnetic field in the transformer core. Through electromagnetic induction, this magnetic field induces a lower voltage in the secondary winding, with the voltage determined by the transformer turns ratio.
Voltage Regulation: The railway transformer adjusts the voltage to match the requirements of the train’s traction system, typically reducing it to a lower voltage level suitable for the train’s propulsion equipment.
Power Delivery: The transformed power is then delivered to the traction system, including the traction motors or other onboard equipment, enabling the train to operate.
Protection & Control: Railway transformers must handle both high input voltage and varying load demands during train operation. Protective devices such as circuit breakers and sensors monitor temperature, voltage, and current to ensure safe and reliable operation.
Feedback & Adjustment: Continuous monitoring allows the system to respond to changes in load and operating conditions, helping maintain stable performance and prevent failures.
Types of Railway Transformers
There are several types of railway transformers, and their designs vary depending on the railway power supply system, installation location, and application. The main types include stationary traction transformers, onboard traction transformers, auto-transformers, and multi-system traction transformers.
Stationary Traction Transformers
Stationary traction transformers are installed at railway substations along the railway line. They step down high-voltage power from the grid or railway power supply network before supplying electricity to the train through the overhead catenary system or third rail.
These transformers are commonly used in railway electrification systems to provide a stable and reliable power supply for electric trains.
Onboard Traction Transformers
Onboard traction transformers are installed directly on electric trains or locomotives. They have a compact and lightweight design because they must move with the train while handling the high-power demands of the traction system.
These transformers receive power from the railway overhead system and step it down to a suitable voltage for the train’s traction and electrical equipment.
Traction Auto-Transformers
Traction auto-transformers are used in railway power supply systems, particularly in high-voltage and high-speed rail applications. They help maintain the required voltage along the railway line and can reduce voltage drop and power losses over long distances.
Unlike conventional two-winding transformers, an auto-transformer uses a common winding for part of the primary and secondary circuits.
Multi-System Traction Transformers
Multi-system traction transformers are designed for electric trains that operate across railway networks with different electrification systems. They can accommodate different supply voltages and frequencies, allowing trains to operate across regions or countries with different railway power standards.
Transformers for AC and DC Traction Systems
Railway traction transformers can also be classified according to the type of railway power supply.
AC traction transformers are used in AC railway electrification systems, including many long-distance and high-speed rail networks. They step down the incoming AC voltage to the level required by the train’s traction system.
DC traction systems are commonly used for metro trains, trams, and suburban railways. In these systems, transformers are generally used together with rectifiers to convert the incoming AC power into the DC power required by the railway system.
The choice of railway transformer depends on the railway electrification system, voltage and frequency, installation type, train requirements, and operating conditions.
Applications of Railway Traction Transformers
Railway traction transformers are mainly used in electrified railway systems to provide reliable power for trains and related railway equipment. Their applications vary according to the type of railway, train, and electrification system.
| High-Speed and Passenger Rail | |
 | Traction transformers are widely used in high-speed and conventional passenger rail systems. They provide the required electrical power for trains operating at high speeds and under frequently changing load conditions. |
| Metro, Light Rail, and Trams | |
 | Traction transformers are also used in urban transit systems, including metro trains, light rail, and trams. They support the electrical power supply required for frequent acceleration, braking, and stop-and-go operation. |
| Freight Railways | |
 | Electric freight trains require traction transformers to support heavy loads and long-distance operation. These applications place high and continuously changing demands on the traction power system, making reliable transformer performance essential. |
| International Rail Systems | |
 | Trains operating across countries or railway networks with different electrification standards may use multi-system traction transformers. These transformers are designed to accommodate different supply voltages and frequencies, allowing trains to operate across multiple railway systems. |
Railway Traction Transformer Design and Specifications
The traction transformer design depends on the railway electrification system, installation type, voltage level, power requirements, and operating conditions. Compared with standard power transformers, railway transformers need to withstand frequent load changes, mechanical vibration, electrical stresses, and demanding operating conditions.
Core and Windings
The transformer core provides a low-reluctance path for magnetic flux between the windings. High-quality core materials and a rigid construction help the transformer maintain reliable performance under fluctuating railway loads.
The windings are designed according to the required voltage and power rating. High-quality conductor materials and insulation systems are used to provide the mechanical strength and electrical insulation required for railway applications.
Voltage and Frequency
The traction transformer specifications depend on the railway power supply system. Common railway supply levels include:
- 15 kV, 16.7 Hz AC – used in some European railway networks.
- 25 kV, 50/60 Hz AC – widely used for railway electrification.
- 1.5 kV DC and 3 kV DC traction systems – used in some urban and conventional railway networks.
The output voltage is selected according to the requirements of the traction system and downstream electrical equipment.
Power Rating
The traction transformer rating is normally specified in kVA or MVA and depends on the train, railway network, and expected load. Different applications may require different power ratings, from smaller units for light rail and trams to higher-capacity transformers for high-speed trains and heavy-duty railway systems.
The transformer must also be designed to withstand short-term overloads and rapid changes in load during train acceleration, braking, and normal operation.
Cooling and Construction
Traction transformers can use different cooling methods depending on the installation and power requirements.
Oil-immersed transformers use insulating liquid for both cooling and insulation.
Dry-type transformers use air for cooling and solid insulation materials, making them suitable for applications where oil is not preferred.
For onboard applications, the transformer must also meet strict space and weight requirements. Depending on the train design, transformers may be installed in machine rooms, under the floor, in low-floor areas, or on the roof.
Tap Changer and Voltage Regulation
A tap changer can be used to adjust the transformer voltage according to the requirements of the railway power system. Depending on the application, a traction transformer may use an on-load tap changer (OLTC) or an off-load tap changer.
An OLTC can adjust the voltage while the transformer remains energized, while an off-load tap changer requires the transformer to be disconnected before the tap position is changed.
Efficiency and Energy Losses
Traction transformers are designed for high efficiency because they operate under frequent and changing load conditions. Their energy losses mainly include core losses and load losses.
- Core losses: Mainly caused by hysteresis and eddy currents in the transformer core. These losses remain relatively stable when the transformer is energized.
- Load losses: Mainly caused by winding resistance and increase with the square of the load current.
- Other losses: Stray losses, dielectric losses, and the power consumed by cooling equipment may also contribute to the total losses.
The actual efficiency and losses depend on the transformer design, rated power, materials, cooling method, and operating load. Therefore, transformer efficiency and loss values should be evaluated according to the specific project requirements and applicable standards.
Protection and Testing
Reliable protection is essential because traction transformers operate under high electrical and dynamic loads. Typical protection and monitoring functions include:
- Overcurrent protection
- Overvoltage protection
- Temperature monitoring
- Insulation monitoring
- Ground fault protection
Traction transformer testing is carried out to verify electrical performance, insulation strength, temperature performance, and overall reliability before the transformer is placed into service.
The final traction transformer specifications and ratings should be determined according to the railway voltage system, frequency, power demand, installation conditions, cooling method, and applicable standards.
Key Design Challenges
Railway traction transformers need to balance several requirements, including:
High power: Handle high and rapidly changing loads.
Thermal management: Dissipate heat under demanding operating conditions.
Mechanical strength: Withstand vibration and mechanical stress.
Compactness: Meet space and weight limitations, especially for onboard applications.
Reliability: Maintain stable operation with minimum maintenance.
How to Select a Railway Transformer?
Selecting the right railway transformer depends on the railway power system, load requirements, installation conditions, and applicable standards. Before placing an order, it is important to confirm the following key specifications.
Rated Power and Voltage
The transformer should have sufficient capacity to handle the expected railway load, including peak and changing loads during operation. The required rated power and high-voltage level should be determined according to the railway power supply system.
SCOTECH can provide transformers with rated power up to 100 MVA and rated high voltage up to 132 kV, with customized ratings available according to project requirements.
Connection and Phase Requirements
The transformer connection should match the railway traction system and its power supply configuration. Different railway applications may require different phase arrangements and winding connections.
Available connection options include:
- YN,d11
- YN,d11,d5
- V₀V₀
- Scott-connected (3-phase/2-phase)
- Leblanc-connected (3-phase/2-phase)
- Woodbridge-connected (3-phase/2-phase)
Insulation and Cooling Medium
The choice of insulating and cooling medium depends on the installation environment, performance requirements, and project specifications.
SCOTECH can provide different options, including FR3, mineral oil, silicone, hydrocarbon, and vegetable oil.
Cooling systems can also be configured according to the transformer rating and application, with galvanized or stainless-steel radiators available.
Tank and Accessories
The transformer tank and cover should provide sufficient mechanical strength and protection for the internal components. Depending on the project requirements, SCOTECH offers mild steel or stainless-steel tanks and covers.
Additional accessories can also be configured, including:
- Buchholz relay
- Current transformer (CT)
- On-load tap changer (OLTC)
- Marshalling box
- Winding temperature indicator
- Other monitoring and protection accessories
Impedance and Custom Requirements
Transformer impedance affects voltage regulation and short-circuit performance, so it should be selected according to the railway system requirements.
SCOTECH supports custom impedance and can provide an electrostatic shield when required. Other transformer parameters can also be customized according to the project specification.
Standards and Testing
Railway transformers should be designed and tested according to the standards required for the specific project and market. SCOTECH designs transformers according to applicable IEC, ANSI/IEEE, NEMA, CSA, and other relevant standards.
Before selecting a railway transformer, confirm the required power rating, voltage, frequency, connection, insulation and cooling medium, impedance, accessories, installation conditions, and applicable standards. Providing these details to the transformer manufacturer helps ensure that the final design matches the railway power system and operating requirements.
